In any communication system, it is desirable to increase the Signal to Noise Ratio (SNR) of the system. This has the advantageous effect of increasing the fidelity of a system for transmitting an analog signal, or decreasing the bit error rate of a system for transmitting a digital signal. Prior art methods for increasing the SNR of a system focus on increasing the available signal power at the input to the receiver, or decreasing the amount of noise in the receiver. The prior art teaches several ways to increase the signal power at the receiver, for instance, one may increase the gain of the RF amplifier in the transmitter, or make antennas larger or more directive. Another way to increase the effective power at the receiver, in the prior art, is to increase the bandwidth used to transmit the signal by spectrum spreading, or less directly, by coding for error correction. Spread spectrum techniques are taught in the books Principles of Communication Systems, Second Edition, by Herbert Taub and Donald L. Schilling, McGraw Hill, 1986, and Spread Spectrum Systems, Second Edition, by Robert C. Dixon, John Wiley & Sons, 1984. One prior art technique for decreasing the amount of noise in the receiver is that of using a receiver with a low-noise amplifier at the front end of the receiver.
Spread spectrum communication systems typically operate with a very low power density spread over a wide enough bandwidth, sometimes known as the chipping bandwidth, to achieve a certain processing gain and, hence, required SNR for a given communication task. The noise in a spread spectrum system is often largely self-interference, as well as additive Gaussian white noise, which appear in the receiver as an interfering random signal having maximum power in the center of the spread spectrum bandwidth, in the portion of that bandwidth in which the desired spread spectrum signal is conventionally received. It is frequently desirable to increase the SNR in a system, especially if this can be achieved with only minimal changes to existing hardware, for instance, without changing antennas or RF amplifiers, and without significantly increasing the power or power density or occupied bandwidth of the transmitted signal. Increasing the SNR of a system significantly increases the capacity of the system. A 3 dB increase in SNR for a system allows an approximate doubling of the number of users that can be supported by the system.